A peak-to-average power ratio (PAPR) is a measure of instantaneous peak power relative to the average power being supplied by a power amplifier when amplifying a given input signal to provide an amplified output signal. The PAPR has an impact on amplifier efficiency, which is invariably an important attribute of portable communication systems that rely on battery power. More efficient amplifiers require less energy to amplify a given signal to a certain level than less efficient amplifiers. Generally, a lower PAPR enables a higher amplifier efficiency, whereas a higher PAPR results in lower amplifier efficiency. Accordingly, designers are constantly trying to build more efficient communication systems that result in lower PAPRs.
The PAPR for communication systems is typically a function of the input signal being amplified by the power amplifier. The peak and average amplitudes of the input signal correlate to the instantaneous peak and average powers provided by the power amplifier while amplifying the input signal. As such, an input signal that has relatively high instantaneous peaks in amplitude with respect to the overall average amplitude is considered a high PAPR signal, whereas an input signal that has relatively low instantaneous peaks in amplitude with respect to the overall average amplitude is considered a low PAPR signal. The peak and average amplitudes of the input signal are often a function of how the input signal is modulated.
Typical modulation schemes employed in modern communication systems include frequency division multiple access (FDMA), including orthogonal frequency division multiple access (OFDM); code division multiple access (CDMA); and time division multiple access (TDMA) schemes. OFDM systems, such as the Third Generation Partnership Project's (3GPP's) Long Term Evolution (LTE) standard and the World Wide Interoperability for Microwave Access (WiMAX) standard, employ a number of independently modulated sub-carriers, which can result in high PAPRs. CDMA systems, such as the Universal Mobile Telecommunications Systems (UMTS), employ spread spectrum modulation and are also considered to have high PAPRs, similar to OFDM systems. TDMA systems, such as Global System for Mobile Communications (GSM) employ a constant power envelope, and as such, have very low PAPRs. Enhanced Data Rates for GSM Evolution (EDGE) is non-constant envelope and generally lies between GSM and CDMA/OFDM systems with respect to PAPR. For systems that have relatively high PAPRs, techniques have been employed to reduce the peak amplitude of the modulated input signal prior to amplification in an effort to reduce the associated PAPR, and as a result, may increase the efficiency of the power amplifier.
An exemplary PAPR reduction technique involves distorting a given input signal, which is only modulated according to a single modulation scheme, to effectively reduce those peaks that exceed a given threshold. Prior to amplification, the peaks of the input signal that exceed the given threshold are removed, or clipped, to form a clipped signal. The clipped signal is subtracted from the input signal to form a distortion signal, which is subsequently processed and applied to the entirety of the input signal to result in peak reduction. Application of the attenuated distortion signal to the input signal effectively reduces those peaks that exceed the given threshold by a desired amount. This and other PAPR reduction techniques have proven relatively successful when applied to signals that are only modulated according to a single modulation scheme.
However, certain communication systems are now being required to process signals using different modulation schemes at the same time. With the evolution from second generation (2G/2.5G) and third generation (3G) networks that rely on CDMA and TDMA schemes to fourth generation (4G) networks that rely on OFDM schemes, communication systems often need to simultaneously support a combination of CDMA and OFDM schemes, TDMA and OFDM schemes, or perhaps CDMA, TDMA, and OFDM schemes. Unfortunately, when the different signals from different modulation schemes are combined, the PAPR associated with the combined input signal can significantly increase. Even if corresponding PAPR reduction techniques are individually applied to each of the different signals before the respective signals are combined, the combined input signal will still have an undesirable PAPR. This is particularly the case when an OFDM signal is combined with a TDMA or CDMA signal. Further, application of existing PAPR techniques to such a combined signal have proven ineffective, primarily because the different types of signals in the combined signal require different PAPR techniques, or one type of signal is intolerant to the PAPR techniques required by another type of signal.
Accordingly, there is a need for an effective and efficient technique to reduce the PAPR associated with a combined signal that includes two or more signals that were generated using different modulation schemes.